For decades, flexible pipes have been serving the offshore industry – not only at floating facilities but also subsea – as an economical alternative to steel piping for flowlines and subsea installations. While their versatility literally extends from shallow to deep- and ultra-deep-water applications for systems and fields, operators face extreme operating conditions such as harsh environments, long distances and extreme temperatures. Our experts have stepped up to the challenge and have been working on developing solutions suited for those applications. Find out how in this article.
The integrity management of flexible pipes is a challenge because of their multi-layer structure consisting of different materials. The inner carcass is typically manufactured from different types of stainless steel or duplex steel, whereas the next pressure armor layer usually consists of carbon steel.
As the leading provider of in-line inspection (ILI) services, ROSEN started the development of ILI technologies for flexible pipes, which can contribute valuable information for the integrity assessment of flexibles. Naturally, the first focus of this two-fold approach would be the inspection of the innermost layer – the carcass as shown in Figure 1 – but in the next stage, the pressure armor layer should come into play as well. If complete regularity of the inner carcass and ideally the pressure armor layer is provided, operators get substantial proof of the integrity of flexibles.
Figure 1 – Typical multi-layer structure of a flexible pipe
Compared to inspection technologies applied from the outside (e.g. crawlers, divers, ROVs), in-line inspections have several benefits: they are relatively easy to deploy, since no additional infrastructure like boats is necessary; the measurement of the full length of any flexible is possible, since there are no restrictions at the end fittings, at buoyancy modules and at touchdown points; and flexible (buried) flow lines can be inspected as well.
ROSEN received several meters of a 6-inch flexible pipe from an operator as a starting point for the development of the technology. Two laboratory samples for carcass and pressure armor were prepared and set up for the initial test measurements. It was decided to start this two-fold approach with the assessment of the inner carcass, because if the full regularity of this layer could be proven, a first module for the integrity assessment of the flexible would be provided. Two tasks had to be executed: first, the inspection for the overall regularity of the inner carcass and, second, the detection of single anomalies such as pitting corrosion, erosion or cracking.
CONTINUOUS DATA FLOW
A valuable piece of information for carcass regularity is the pitch between the single elements or structures on the inside. To achieve optimal accuracy, we decided to develop a high-resolution eddy current measurement system, because such a system provides continuous data rather than only single data points like other systems. Figure 2 shows one carcass laboratory sample and a high-resolution eddy current measurement of the carcass inside, with the typical lamella-like pattern divided by small “notches.” This pattern is clearly present in the data. The gray line shows a measurement track that was evaluated for the calculation of the carcass pitch. This data of a single track or channel is shown in Figure 3. The continuous character of the eddy current data is evident. The carcass pitch can now be defined as the distance between single minima or single maxima in the data – or both. Over a length of 250 mm, the mean pitch was evaluated and the standard deviation calculated for both cases. As reference, a caliper gauge measurement was used, as summarized in Table 1.
Figure 2 – Carcass laboratory sample measured with a high-resolution eddy current system. The typical pattern is clearly present in the data. The gray line marks a measurement track that is used for pitch calculation
Figure 3 – Carcass eddy current measurement along the gray line in Figure 2. As carcass pitch, the distance between either single minima or maxima could be defined
Table 1 – Evaluation of a measurement track with a length of 250 mm; the mean pitch is calculated for the distance of the measurement maxima as well as for the measurement minima and compared to a reference caliper gauge measurement
The calculated mean values for the carcass pitch only deviate about several hundredths of a millimeter from the caliper gauge reference measurement. The standard deviation of the evaluated 250 mm is somewhat larger for the maxima distance but very accurate for the minima distance of about 0.12 mm, on the same level as the reference measurement. And despite that, at this point of the technology development, it is unclear how large the manufacturing tolerances of the carcasses of flexibles actually are; the measurements show excellent results and hint at the potential of high-resolution eddy current measurements.
DEVELOPMENT OF A NEW SENSOR CARRIER FOR ILI TOOLS
Based on the laboratory test, a new sensor carrier for in-line inspection tools could be developed. A high-resolution eddy current sensor system was combined with a very sensitive magnetic flux leakage sensor system to inspect the inner carcass and to create the potential for a pressure armor inspection as well. A prototype ILI-tool was equipped with the new sensor carriers, and qualification pull tests were conducted (Figure 4).
Figure 4 – Qualification pull test of the newly developed sensor for flexible pipe inspection. The data example shows the front-end fitting with the subsequent typical carcass pattern
Excellent data could be gathered from both end fittings and from the carcass, enabling the evaluation of the carcass pitch as shown for the laboratory measurements. Figure 5 shows the calculated carcass pitch for one eddy current channel over a length of more than 10 meters. A mean pitch of 20.33 mm with a standard deviation of 0.26 mm (1.28%) could be achieved; this means the pitch can be measured with submillimeter accuracy. With the new sensor, it will be possible to assess with reasonable accuracy the overall regularity of the inner carcass of flexibles via the data, as shown in the C-scan, and via the calculated pitch values. Hence, the first of our tasks could be accomplished.
DETECTING DEFECTS WITHIN THE CARCASS
Regarding the ability of the new sensor system to detect possible anomalies or flaws in the carcass, different artificial defects were manufactured in the laboratory samples, and measurements were conducted. The data example in Figure 6 shows the clear detection of axial crack-like anomalies as well as the detection of pitting and erosion-type anomalies. Another specimen with circumferential crack-like anomalies was tested, showing convincing detection capabilities as well. On the laboratory scale, the second task for the successful inspection of the inner carcass could be completed as well. In parallel to the flexible pull test, the magnetic flux leakage part of the new sensor was qualified in compliance with the ROSEN standard magnetic flux leakage tool fleet. The upshot: carbon steel pipelines, which are often connected to flexible risers, can also be inspected with the new sensor system in a single inspection run.
Figure 5 – Calculated carcass pitch along a measurement track in the flexible pull test. Submillimeter accuracy could be achieved
Figure 6 – Laboratory measurement of artificial defects in the carcass laboratory sample with the new sensor shows axial crack-like defects; pitting corrosion-type and erosion-type defects were manufactured and could clearly be detected
TAKING THE DEVELOPMENT FURTHER
Looking forward to the next stage in the development of the technology, first magnetic measurements of the pressure armor layer showed promising results. Very similar to the carcass inspection, it was also possible to calculate the “pitch” of the pressure armor layer since the typical spiral pattern showed up clearly in the laboratory data. Further options include applying the capabilities of the high-resolution eddy current system to the detection of internal cracks in other non-standard pipes made from duplex or cladded with stainless steel, i.e. where conventional tools are not suitable. The ROSEN laboratory setup is currently being adapted for just those kinds of test measurements.